1. Field of the Invention
This invention relates to the design and fabrication of magnetic tunnel junctions (MTJ) as memory storage devices, particularly to a design wherein word and bit lines are placed very close to the MTJ and cladded with magnetic material so as to produce higher magnetic flux at the MTJ free layer for a given write current.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of magnetized ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, however, because it depends on the availability of tunneling states that accept electrons with different spin orientations. Therefore, the overall tunneling current will depend on the number of spin-up vs. spin-down electrons, which in turn depends on the orientation of the electron spin relative to the magnetization direction of the ferromagnetic layers. Thus, if the relative magnetization directions are varied for a given applied voltage, the tunneling current will also vary. As a result of this behavior of an MTJ, sensing the change of tunneling current for a fixed voltage can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers can be varied relative to the other and also that changes in these relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel (low resistance) or antiparallel (high resistance) configurations, when writing data, and of having its tunneling current variations or resistance variations measured, when reading data.
In practice, the free ferromagnetic layer of the MTJ can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the fixed layer may be thought of as being permanently aligned in its easy axis direction. When the free layer is anti-aligned with the fixed layer, the junction will have its maximum resistance, when the free layer is aligned with the fixed layer, the minimum resistance is present. In typical MRAM circuitry, the MTJ devices are located at the intersection of orthogonal current carrying lines called word lines and bit lines. When both lines are carrying current, the device is written upon by having the magnetization direction of its free layer changed. When only one line is carrying current, the resistance of the device can be sensed, so the device is effectively read. Note that an additional current carrying line may be present in some device configurations to sense the resistance of the device, but in simplest terms the device behaves as described above. Such an MTJ device is provided by Gallagher et al. (U.S. Pat. No. 5,650,958), who teach the formation of an MTJ device with a pinned ferromagnetic layer whose magnetization is in the plane of the layer but not free to rotate, together with a free magnetic layer whose magnetization is free to rotate relative to that of the pinned layer, wherein the two layers are separated by an insulating tunnel barrier layer.
In order for the MTJ MRAM device to be competitive, in terms of power consumption and device density, with other forms of DRAM, it is necessary that the MTJ be made very small, typically of sub-micron dimension. Parkin et al. (U.S. Pat. No. 6,166,948) teaches the formation of an MTJ MRAM cell in which the free layer is formed of two antiparallel magnetized layers separated by a spacer layer chosen to prevent exchange coupling but to allow direct dipole coupling between the layers. The free layer thereby has closed flux loops and the two layers switch their magnetizations simultaneously during switching operations. Parkin notes that sub-micron dimensions are needed to be competitive with DRAM memories in the range of 10–100 Mbit capacities. Parkin also notes that such small sizes are associated with significant problems, particularly super-paramagnetism, which is the spontaneous thermal fluctuation of magnetization in samples of ferromagnetic material too small to have sufficient magnetic anisotropy (a measure of the ability of a sample to maintain a given magnetization direction). To overcome the undesirable spontaneous thermal fluctuations in MRAM cells with very small cross-sectional areas, it is necessary to make the magnetic layers thick. Unfortunately, the size of the required switching field increases with layer thickness, so the price paid for a thermally stable cell is the necessity of expending a great deal of current to change the magnetic orientation of the cell's free layer.
Some degree of anisotropy is necessary if an MTJ cell is to be capable of maintaining a magnetization direction and, thereby, to effectively store data even when write currents are zero. As cell sizes have continued to decrease, the technology has sought to provide a degree of magnetic anisotropy by forming cells in a wide variety of shapes (eg. rectangles, diamonds, ellipses, etc.), so that the lack of inherent crystalline anisotropy is countered by a shape anisotropy. Yet this form of anisotropy brings with it its own problems. A particularly troublesome shape-related problem in MTJ devices results from non-uniform and uncontrollable edge-fields produced by shape anisotropy (a property of non-circular samples). As the cell size decreases, these edge fields become relatively more important than the magnetization of the body of the cell and have an adverse effect on the storage and reading of data. Although such shape anisotropies, when of sufficient magnitude, reduce the disadvantageous effects of super-paramagnetism, they have the negative effect of requiring high currents to change the magnetization direction of the MTJ for the purpose of storing data.
One way of addressing the problem of the high currents needed to change the magnetization direction of a free layer when its shape anisotropy is high, is to provide a mechanism for concentrating the fields produced by lower current values. This approach was taken by Durlam et al. (U.S. Pat. No. 6,211,090 B1) who teach the formation of a flux concentrator, which is a soft magnetic (NiFe) layer formed around a copper damascene current carrying line. The layer is formed around three sides of the copper line which forms the digit line at the underside of the MRAM cell.
Deak (U.S. Patent Application Publication No.: U.S. 2003/0207486 A1) discloses an array of MRAM devices formed between word and bit lines that are clad with low remanence (remaining B field after the inducing H field has been reduced to zero) flux concentrators. The remanence thereby serves an advantageous purpose of providing the cell with a biasing magnetic field.
Sharma et al. (U.S. Pat. No. 6,593,608 B1) provides a magnetic memory device having two soft magnetic reference layers (fixed layers) and two barrier layers, wherein the two barrier layers are formed on the soft magnetic reference layers which also play the role of cladding layers.
Goronkin et al (U.S. Pat. No. 6,525,957 B1) provides a MRAM type multi-state memory cell in which magnetic flux is confined within the cell by forming the cell around the current carrying line.
Jones et al. (U.S. Patent Application Publication No.: U.S. 2003/0151079 A1) forms a magnetically clad bit line structure in which the bit line is formed within a trench whose sidewalls contain the magnetic cladding.
This invention addresses the problem of the high current required to reorient the magnetization of the free layer in ultra-small MRAM cells wherein the super-paramagnetic behavior requires thick free layers. It does so by forming composite, ultra-thin word and bit lines, each line of thickness less than 100 nm, each line with an adjacent soft magnetic layer that concentrates the magnetic field at the free layer and each line formed on the same side of the free layer.